(1) Field of the Invention
The present invention relates to methods for the microencapsulation of an agricultural active having a high melting point, and more particularly to methods for the microencapsulation of such material while maintaining the temperature of the agricultural active below its normal melting point and without contacting the active with an aromatic solvent.
(2) Description of the Related Art
Materials that affect the growth and development of agronomically important plants, or that provide some type of protection of plants from pests and diseases, are commonly referred to as agricultural actives. Such materials are widely used in modern agriculture and provide benefits of increased yield, vigor and overall plant health. However, some agricultural actives have harmful effects if they are ingested or otherwise contacted by humans or other animals.
The most common method for using agricultural actives is direct application of the active to a plant, seed, or to the soil in which the plant is to be grown. But wind, runoff and groundwater leaching can cause undesirable movement of the active, which can result in its unintended contact with plants and animals in streams, neighboring fields and homes. Furthermore, such movement of the active from the zone where protection is desired can result in reduction of the concentration of the active to below efficacious levels by the time the target pest arrives. Irreversible binding of the active to components of soil exacerbates this problem.
One method to improve the delivery and safety characteristics of agricultural actives is to include them as components of a controlled release composition. Many different types of such controlled release compositions are known and include the encapsulation of droplets, the formation of a coating on solid particles, and the inclusion of actives in matrix microparticles. Such formulations typically place coatings of a barrier material between the active and the environment through which the active must move in order to reach the environment. The rate at which such transfer takes place depends upon the type of coating, its thickness, the chemical affinity of the active for the matrix, as well as many environmental parameters, such as temperature, moisture levels, and the like.
Thus, encapsulation of an active can improve its safety and stability, its dispersibility and even distribution, its handling characteristics, as well as to control its release rate.
Although both solid particles and liquids can be enclosed in coatings, the formation of microcapsules around liquid droplets is believed to have several advantages over the coating of solid particles. For example, microcapsules formed around liquid droplets have regular spherical geometry, coatings of even thickness, and lack sharp edges and concave surfaces, which can occur in coatings of solid particles, and which could cause uneven coating thickness or even lack of a coating on some parts of the active. A coating having a regular geometry and an even thickness provides more predictable release characteristics than an uneven coating of varying thickness.
A number of methods for the encapsulation of liquid droplets containing agricultural actives is known in the art, and a summary of such methods is provided, for example, in Controlled-Release Delivery Systems for Pesticides, H. B. Scher, Ed., Marcel Dekker, Inc., New York (1999); in Microencapsulation, S. Benita, Ed., Marcel Dekker, New York (1996); and in Microencapsulation and Related Drug Processes, Patrick B. Deasy, Ed., Marcel Dekker, New York (1984).
Matson, in U.S. Pat. Nos. 3,516,846 and 3,516,941 and Sher et al., in U.S. Pat. No. 4,956,129, describe the formation of a urea-formaldehyde polymer coating around small liquid droplets.
Another commonly used method for the encapsulation of liquid droplets involves the generation of a polyurea shell around an active-containing core by interfacial polymerization at the surface of the droplets. Advantages of using a polyurea shell include that the material is generally non-phytotoxic, its permeability characteristics can be controlled, and the shell can be formed at relatively low temperatures—in fact, polymerization temperatures of lower than 90° C. are almost always used, and temperatures of from about 40° C.-70° C., are preferred.
In U.S. Pat. Nos. 4,285,720 and 4,643,764, Scher describes a process involving the blending of various pesticides with an organic polyisocyanate to form an organic phase, which is dispersed into small droplets into an aqueous phase. Some molecules of the organic polyisocyanate hydrolyze to form amines, which then can react with other isocyanates to form the polyurea shell.
Chao, in U.S. Pat. No. 4,599,271, describes the use of two or more organic-in-aqueous emulsions for the formation of a polyurea shell around a polyisocyanate containing droplet.
Beestman (in U.S. Pat. No. 4,640,709) discloses the inclusion of an alkylated polyvinyl pyrrolidone polymer that acts as an emulsifier in the aqueous phase of a two-phase system which is capable of providing microcapsules having high levels of an enclosed water-immiscible material.
In U.S. Pat. No. 4,681,806, Matkan et al. describe particles containing a releasable fill material and having a polyurea surface layer that encloses a polyurea matrix having the fill material contained therein.
Ohtsubo et al. (in U.S. Pat. No. 4,889,719) describe the microencapsulation of organophosphorous insecticidal compositions by the formation of a polyurea shell. Similar methods have been used for the microencapsulation of herbicidally active N-chloroacetylcyclohexeneamines and herbicidally active chloroacetanilide in a polyurea shell, and are described in U.S. Pat. No. 5,006,161, to Hasslin et al.
In U.S. Pat. No. 4,738,898, Vivant describes microencapsulation of a variety of materials within polyurea skin membranes by interfacial polyaddition involving a polyisocyanato hydrophobic liquid in an essentially aqueous medium. The polyisocyanato hydrophobic liquid contained the dissolved material to be encapsulated, an aliphatic diisocyanate and an isocyanurate ring trimer of an aliphatic diisocyanate. The isocyanate materials were reacted with a polyamine to form a polyurea shell material. The microcapsules described by Vivant had leakproof walls that were designed for the microencapsulation of colorants and the production of pressure-sensitive carbonless paper, for example. The microcapsules were designed to maintain the encapsulated material until the capsule was ruptured, and would not have been suitable for the controlled release of the encapsulated materials through the walls of the capsule.
Hasslin et al. (in U.S. Pat. No. 4,938,797) describes the encapsulation of a water-immiscible pesticide in a polyurea shell. The method includes the use of an anionic dispersant, such as a salt of polystyrenesulfonic acid in the aqueous phase. A similar method is described in U.S. Pat. No. 5,310,721, to Lo, but all of the agricultural active materials that are encapsulated are liquids at ambient temperature.
Seitz et al. (in U.S. Pat. No. 5,925,595) disclose a process for the preparation of microencapsulated materials—including low-melting herbicides, such as acetanilides—by combining a triisocyanate and a diisocyanate with a water immiscible composition which can include the herbicide; forming a dispersion of the core chemical and the blend of isocyanates in an aqueous liquid; and reacting the isocyanates with a polyamine to form microcapsules.
In U.S. Pat. No. 6,133,197, Chen et al., describe the formation of quick release microcapsules containing an agriculturally active material and having a polyurea shell with relatively low degree of cross-linking.
Despite the advantages provided by microcapsules having polyurea shells that enclose liquid cores containing agricultural actives, the methods that are known for the formation of these structures have certain limitations that limit their use with certain highly promising actives. For example, if the agricultural active has a melting point that is close to, or higher than, the preferred range of polymerization temperature for polyurea shell formation, it is difficult to liquify the active in order to form the microcapsule. A common solution to this situation has been to dissolve the active in an aromatic solvent. See, e.g., WO 00/27200, where the formation of a slow release capsule suspension is described wherein a mixture of a fungicide (thienol[2,3-d]pyrimidin-4-one) and another agricultural active (selected from a list of possible materials) is blended with polyisocyanates in an aromatic solvent. This organic phase is emulsified in an aqueous liquid phase, and 1,6-diaminohexane is added to cause a polymerization reaction with the isocyanates to form microcapsules that enclose the mixture of agricultural actives.
Since many aromatic solvents are phytotoxic, their use in controlled release formulations intended for application to plants or seeds would appear to be potentially harmful to the plant.
One new class of agricultural actives that appears to be very promising for fungicidal and other applications is described in U.S. Pat. Nos. 5,482,974, 5,486,621, 5,498,630, 5,693,667, 5,693,667, 5,705,513, 5,811,411, 5,834,447, 5,849,723, 5,994,270, 5,998,466, 6,028,101, and in publications WO 93/07751, and EP 0 538 231 A1. One such compound, in particular, is 4,5-dimethyl-N-(2-propenyl)-2-(trimethylsilyl)-3-thiophenecarboxamide, having a CAS registration number of 175217-20-6, and for which the proposed ISO common name is “Silthiopham”. Silthiopham has a normal melting point of about 86° C.-88° C., which has limited its incorporation into polyurea microcapsules by known techniques. Further information about silthiopham can be found in U.S. Pat. No. 5,486,621.
Accordingly, it would be useful to provide a method for the formation of microcapsules enclosing such high-melting agricultural actives where the method was free of the use of aromatic solvents—and preferably free of any solvents—and where the method could be carried out at a temperature that was below the normal melting point of the active. It would also be useful if such method allowed for the use of a polyurea shell that could be designed to release the active from the microcapsule at a controlled rate when the microcapsule was exposed to natural environmental conditions.